It is often the case that a lightweight aluminum alloy plate, etc. are used for an outer plate and the like applied to a fuselage of a commercial aircraft, and pocket machining (thin-wall machining) is performed thereon to further reduce the weight. Pocket machining refers to formation of multiple pockets (recesses) on the inner surface of the outer plate by machine cutting or chemical milling (etching). However, besides being costly and taking a long machining time, chemical milling is becoming difficult to apply due to recent environmental regulations, since the aluminum alloy removed from the pocket is dissolved in a chemical solution and becomes a large amount of waste liquid, which cannot be recycled. Therefore, a plate-shaped workpiece forming method which can efficiently machine the pocket by machine cutting has been explored.
Since the outer plate constituting the fuselage of the aircraft has a curved shape with a predetermined curve radius, the outer plate is curved before or after the aforementioned pocket machining. Conventionally, there have been a method called bending-first forming method of performing pocket machining as post-machining after curving the plate-shaped workpiece intended for the outer plate, and a reverse method called bending-later forming method of curving the plate-shaped workpiece after performing pocket machining thereon.
An advantage of the bending-later forming method is that, since pocket machining is performed on the plate-shaped workpiece in a flat state, the pocket can be machined efficiently by machine cutting. On the other hand, as the plate-shaped workpiece is curved after the pocket has been machined and the plate thickness has become uneven, it tends to be curved into a polygonal shape. To prevent this, it is necessary to stuff a shim material into the multiple recesses made by pocket machining to achieve an apparently even plate thickness before curving, and significant cost and labor have been expended on the manufacturing and fitting of such a shim material.
Further, the bending-first forming method includes a method, as disclosed in Patent Literature 1, of machining the pocket by machine cutting the curved plate-shaped workpiece in a curved state as it is, and a method of machining the pocket by machine cutting the curved plate-shaped workpiece in a state where the curved plate-shaped workpiece is temporarily spread flat and pressed down.